The present invention relates generally to video processing systems and, more particularly, to time base correction in video processing systems used in video cassette recorders (VCR's) for the recording and playback of television signals.
Time base correction is generally required in video processing systems. See for example, Television Engineering Handbook, K. Blair Benson, Editor in Chief, McGraw-Hill Book Company, New York; 1986, Section 15.5. In advanced digital VCR systems utilizing advanced processing, a concomitant need also arises for clocks used in a time base corrector for digitizing the video information. Such a clock is required to be line locked and to provide an output signal exhibiting a high signal to noise ratio and moreover, it should be able to track fast time changes in the input synchronizing signal. That is, it must exhibit a fast transient response. Thus, in a system utilizing digital processing, a clock for an A/D converter can typically be derived from the horizontal sync rate frequency, which may exhibit jitter on playback from a VCR, so that the output signal of the A/D converter exhibits similar jitter. This output can then be corrected for jitter in a time-base corrector wherein the write or input clock exhibits similar jitter and the read or output clock is relatively free of jitter so as to result in a substantially jitter-free output signal. It is also desirable that the arrangement used should utilize relatively few parts and be capable of realization in integrated circuit (IC) form.
Methods available for generating such clock signals from a source such as the horizontal sync signal include the use of a single phase locked loop (PLL), a multiple PLL and, it is herein recognized, frequency multiplier arrangements. A VCR typically includes a phase locked loop operating at 2.517 MHz, which corresponds to 160 fm (160 times the horizontal frequency). For example, such a frequency is utilized for generating four phase signals at the 629 kHz color signal frequency in a color-under system in a typical VHS type of VCR. The time base corrector typically requires a frequency of 10.07 MHz (640 fm). Thus, for example, in a VCR bandwidth compression system such as that described in U.S. patent application Ser. No. 07/569,029, filed Aug. 17, 1990 in the names of Strolle et al. entitled AN IMPROVED VIDEO SIGNAL RECORDING SYSTEM, a luminance spectrum is "folded" about a folding frequency of about 5 MHz. The Nyquist frequency for sampling this signal is at twice this value. In order to obtain a signal at 10.07 MHz, a phase locked oscillator can be synchronized to the 2.517 MHz signal of the above-mentioned PLL. However, this approach requires considerably more parts which would also add to the cost of VCR's that do not require advanced signal processing. Furthermore, the jitter characteristics of the signal would be affected by the characteristics of the PLL, and more particularly, the loop transient response.